Autogenously crack-healing of concrete has been recognized in the art. Mainly micro-cracks with widths typically in the range of 0.05 to 0.1 mm have been observed to become completely sealed particularly under repetitive dry/wet cycles. The mechanism of this autogenously healing is mainly due to secondary hydration of non- or partially reacted cement particles present in the concrete matrix. Due to capillary forces water is repeatedly drawn into micro cracks under changing wet and dry cycles, resulting in expansion of hydrated cement particles due to the formation of calcium silicate hydrates and calcium hydroxide (Portlandite). These reaction products are able to completely seal cracks provided that crack widths are small. Larger sized cracks can only be partially filled due to the usually limited amount of non-reacted cement particles present. In the latter case healing activity is insufficient as it results only in the formation of a thin layer of hydration products on the crack surface. Besides secondary hydration, also the process of carbonation can contribute to the crack-sealing capacity of commonly applied concrete. This reaction is also expansive, as ingress atmospheric carbon dioxide (CO2) reacts with calcium hydroxide (Portlandite) particles present in the concrete matrix to yield various calcium carbonate minerals such as calcite, aragonite and vaterite.
From a durability perspective, rapid sealing of particularly freshly formed surface cracks is important as this process can substantially delay the ingress of water and other aggressive chemicals into the concrete matrix and thus prevent early material degradation. Several chemicals such as sulphate, chloride and acids are known to dramatically increase concrete matrix degradation and corrosion of embedded steel reinforcement causing a serious threat to the materials performance and durability. One possibility to improve the self-healing capacity of cementious materials is by decreasing the water/cement ratio of the original mixture. A substantial increase in the relative amount of cement or binder in the mixture results in formation of a self-healing buffer, i.e. the presence of a significant amount of non- or only partially reacted binder particles present in the material matrix. Typical examples of such low water to binder ratio types of concrete are high strength or high performance concretes. As recent studies have shown, such concretes do indeed possess a superior crack-sealing capacity compared to ordinary concretes characterized by higher water to cement ratio's.
However, from an environmental viewpoint the latter concrete types (i.e. ordinary types) are preferred as less cement per concrete volume is used. The lower the amount of cement in concrete the lower the environmental pressure in terms of atmospheric CO2 emissions. Although high strength concrete allows building of more slender structures than ordinary concrete and thus need less concrete volume, the total amount of cement used is still significantly higher due to the inherent high percentage of non- or partially hydrated cement particles in the material matrix. The development of a self-healing mechanism in concrete that is based on a potentially cheaper and more sustainable material than cement could thus be beneficial for both economy and environment.
Although bacteria, and particularly acid-producing bacteria, have been traditionally considered as harmful organisms for concrete, recent research has shown that specific species such as ureolytic and other types of calcite-producing bacteria can actually be useful as a tool to repair surface cracks in concrete. In some studies bacteria were externally and manually applied on the concrete surface. Species from the Bacillus group appear promising intrinsic agents as their spores, specialized thick-walled dormant cells, have been shown to be viable for over 200 years under dry conditions. Such bacteria would comprise one of the two components for the envisioned autogenous healing system.
For crack repair filler material is needed, and bacteria can produce that by metabolic conversion of a suitable organic component. The nature of metabolically produced filler material could be bio-minerals such as calcite. These calcium carbonate based minerals are relatively dense and can block cracks, and thus hamper ingress of water efficiently, as was previously demonstrated.
One particular challenge in the development of self healing materials is the need to incorporate sufficient healing agent in the material matrix. As the healing capacity, i.e. the volume of cracks that can potentially be filled may directly be related to the amount of precursor material present, a substantial volume of the material needs to be reserved in order to obtain a significant healing potential. While the matrix-incorporated bacteria function as catalyst and therefore need only a limited volume, it is typically the mineral precursor compound, the second component of the healing system, which will occupy a substantial volume when a significant healing capacity is needed. Particularly for larger cracks to become completely sealed, bulky internal reservoirs or alternatively an intrinsic transportation mechanism is needed. In concrete the latter could be provided by the water-filled continuous capillary pore system which is usually present. The mineral precursor compound could be present in dissolved state in the matrix pore water without affecting strength properties of the material what presumably occurs when specific internal healing agent containing reservoirs are needed. In any case however, incorporated bacteria and the mineral precursor compound should compromise concrete strength properties only to an acceptable extend.
WO2009093898 (EP2082999) describes a healing agent in cement-based materials and structures, wherein said healing agent comprises organic compounds and/or bacteria-loaded porous particles, which porous particles comprise expanded clay- or sintered fly ash. Furthermore, said porous particles are intact spheres, broken or crushed particles derived from said intact spheres, having a specific density between 0.4 and 2 g cm−3. WO2009093898 also describes a process for the preparation of the healing agent.
Jonkers et al. describe in Ecological Engineering 36 (2010) 230-235 an investigation to the potential of bacteria to act as self-healing agent in concrete, i.e. their ability to repair occurring cracks. A specific group of alkali-resistant spore-forming bacteria related to the genus Bacillus was selected for this purpose. Bacterial spores directly added to the cement paste mixture remained viable for a period up to 4 months.
Further, Jonkers et al. describe in Concrete Repair, Rehabilitation and Retrofitting II, Alexander et al. (eds) 2009, Taylor & Francis Group, London, ISBN 978-0-415-46850-3 the development of a two-component self-healing system which is composed of bacteria which catalyse the metabolic conversion of organic compounds to calcite. Both components were mixed with the fresh cement paste, thus becoming an integral part of the concrete. Experimental results seemed to show that ingress water channel through freshly formed cracks activate present bacteria which through metabolic conversion of organic mineral-precursors compounds produce copious amounts of calcite.